The steel referred to as high-speed steel has a micro-structure containing two forms of primary carbide. One is a complex carbide called M.sub.6 C or M.sub.2 C, whose crystal structure constitutes cubic system with a composition of Fe.sub.3 (W, Mo).sub.3 C or Fe.sub.4 (W, Mo).sub.2 C. The other is a mono carbide called MC with a composition of (V, Ti, Nb)C. The former is formed as herringborn-like or feather-like eutectic carbides obtained in eutectic reaction during solidification process of molten steel where austenite (.gamma.) and M.sub.6 C (M.sub.2 C) type carbides are simultaneously crystallized from melt (L). In case of the latter, crystallization style is a little complicated: The MC type carbide may be formed in two solidification types, one is crystallized alone in the melt (L) and the other is formed during eutectic reaction. The MC type carbide crystallized alone Is first formed as a single type crystal from the melt (L) in solidification process. Then, in the eutectic reaction where austenite (.gamma.) and MC type carbide are simultaneously crystallized from the melt (L), MC type carbide may be formed again.
In case of ordinary high-speed steel, M.sub.6 C (M.sub.2 C) type carbide, which is an eutectic carbide, is generated much more than MC type carbide, which is crystallized alone, of the above primary carbides. Besides, M.sub.6 C (M.sub.2 C) type carbides are always generated in eutectic reactions under general industrial conditions for ingot making and cannot be crystallized alone. According to Steven (G. S. Steven, A. E. Nehrenberg: Trans ASM57(1967) p.925), the eutectic temperature here can be expressed in weight % of the elements as shown below: EQU Formula: TM.sub.6 C (.degree.F)=2310-200(% C)+40(% V)+8(% W)+5(% Mo)
When the crystallization temperature difference between MC type carbide and the M.sub.6 C(M.sub.2 C) type eutectic carbides is expressed as .DELTA.T(.degree.C.), the more V, Si, N and C are contained and the less W and Mo are included in a steel, the larger the difference .DELTA.T(.degree.C.) becomes. In general, higher .DELTA.T (.degree.C.) involves more coarse MC type carbide, which lowers steel grindability.
To provide the structure with a finer MC type carbide, it is proposed to decrease the crystallization temperature difference between MC type carbide and M.sub.6 C (M.sub.2 C) type carbide by adjusting the alloying elements (Electric Steel Making, vol. 55, No. 4, 1984, p.225).
According to a conventional method to improve the grindability of high-speed steel, MC type carbide forming elements such as Nb, Ta and TI are added only by a limited amount so as to have finer MC type carbides and content of N is reduced for crystallization of MC type carbides at a lower temperature. This method is to minimize the crystallization temperature difference between MC type carbide and M.sub.6 C or M.sub.2 C type eutectic carbide and thereby prevent coarsening of MC type carbide. Further, addition of rare earth elements such as Ce for combination with N is known to have a similar effect.
However, even when coarsening of MC type carbide is prevented by the above method, it is still inevitable that primary carbide M.sub.6 C or M.sub.2 C is generated as a eutectic carbide.
Primary carbides generated in eutectic reaction are formed into a combined network style during casting process and have a continuous irregular shape. To obtain an excellent member for plastic working with superior mechanical properties from a steel ingot having such a structure, it is important to destroy eutectic carbides by hot working or other means so as to form granular crystals. However, if the forging ratio is not sufficient in relation to product dimensions in forging process, a stripe (streak, hook) structure with crowded distribution of eutectic carbides is generated in longitudinal direction of forged material. Such distribution may cause anisotropy in mechanical properties of the product.
In addition, eutectic carbides cannot be made into solid solution during the subsequent soaking process. Segregation of stripe carbide during hot or cold working is not solved yet.
If such a product is used as a member for plastic working, cracks may occur from the interface between the primary carbides and the matrix, which deteriorates mechanical properties. To solve this problem with improving toughness of the member at the same time, it is effective to reduce the amount of primary carbide, to have finer carbide and to prevent crowded distribution of the primary carbide in longitudinal direction of the forged material. For this purpose, "matrix high-speed steel" with lower amount of primary carbide and powder high-speed steel with micro-sized primary carbide are widely used. However, the former has only a low hardness and involves insufficient absolute values of mechanical properties when used to produce large diameter materials where forging ratio is not sufficient. The latter costs too high and it is difficult to be used popularly.